Electronic Mail in the Internet

Electronic mail has been around since the beginning of the Internet. It was the most popular application when the Internet was in its infancy [Segaller 1998], and has become more and more complicated and powerful over the years. It remains one of the Internet's most important and utilized applications.

As with ordinary postal mail, e-mail is an asynchronous communication medium - people send and read messages when it is convenient for them, without having to coordinate with other people's schedules. In contrast with postal mail, electronic mail is fast, easy to distribute, and inexpensive. Modern e-mail has many powerful features. Using mailing lists, e-mail messages and spam can be sent to thousands of recipients at a time. Modern e-mail messages frequently include attachments, hyperlinks, HTML-formatted text, and photos.

In this section we study the application-layer protocols that are at the heart of Internet e-mail. But before we jump into an in-depth discussion of these protocols, let's take a high-level view of the Internet mail system and its main components.

The following Figure 1 presents a high-level view of the Internet mail system. We see from this diagram that it has three main components: user agents, mail servers, and the Simple Mail Transfer Protocol (SMTP).

We now explain each of these components in the context of a sender, Alice, sending an e-mail message to a recipient, Bob. User agents allow users to read, reply to, forward, save, and compose messages. (User agents for electronic mail we sometimes called mail readers, though we usually avoid this term in this blog.) When Alice is finished composing her message, her user agent sends the message to her mail server, where the message is placed in the mail server's outgoing message queue. When Bob wants to read a message, his user agent retrieves the message from his mailbox in his mail server. In the late 1990s, graphical user interface (GUI) user agents became popular, allowing users to view and compose multimedia messages. At present, Microsoft's Outlook, Apple Mail, and Mozilla Thunderbird are among the popular GUI user agents for e-mail. There are also several text-based e-mail user interfaces in the public domain (including mail, pine, and elm) as well as Web-based interfaces, as we will see soon.

Mail servers form the core of the e-mail infrastructure. Each recipient, such as Bob, has a mailbox located in one of the mail servers. Bob's mailbox manages and maintains the messages that have been sent to him. A usual message starts its journey in the sender's user agent, travels to the sender's mail server, and travels to the recipient's mail server, where it is deposited in the recipient's mailbox. When Bob wants to access the messages in his mailbox, the mail server containing his mailbox authenticates Bob (with usernames and passwords). Alice's mail server must also deal with failures in Bob's mail server. If Alices server cannot deliver mail to Bob's server, Alice's server holds the message in a message queue and attempts to transfer the message later. Reattempts are frequently done every 30 minutes or so; if there is no success after many days, the server removes the message and notifies the sender (Alice) with an e-mail message.

SMTP is the principal appIication-layer protocol for Internet electronic mail. It uses the reliable data transfer service of TCP to transfer mail from the sender's mail server to the recipient's mail server. As with most application-layer protocols, SMTP has two sides: a client side, which executes on the sender's mail server, and a server side, which executes on the recipient's mail server. Both the client and server sides of SMTP run on every mail server. When a mail server sends mail to other mail servers, it acts as an SMTP client. When a mail server receives mail from other mail servers, it acts as an SMTP server.

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